When you build something like a redstone farm, you maybe designed it with expensive support blocks (like iron blocks), and when you want to rebuild it in survival, you maybe want to change those blocks to like stone or wood, and this feature would help a lot !

SscematicRebuildReplaceAll is what you are looking for i belive. just give it a hotkey in hotkeys then hold the key while having the replacement item in your hand, then just right click

The Schematic Rebuild mode (which is a confusing name, it will get renamed to Schematic Edit mode) allows you to do that. See https://github.com/maruohon/litematica/wiki/schematic_editing for more information.

There are various improvements still planned for that mode/functionality though, such as being able to just switch block types via a simple GUI, while maintaining all the block state properties.